Low diffuse scatter, anechoic chamber absorber

ABSTRACT

An electromagnetic chamber absorber provided improved absorption across a wideband and both lower diffuse and specular scatter and a method for constructing the same. An exemplary device can compromise a periodic arrangement of disconnected electromagnetically lossy elements where the periodicity of the lattice is adjusted to suppress all or most grating lobe scattering. Because the electromagnetically lossy elements are disconnected, scalable manufacturing approaches are enabled. The lossy elements can be easily fabricated via shaping, which includes rolling, folding and cutting resistive and/or magnetic sheet materials. The lossy elements can be repeatably placed in a periodic lattice using low density scaffolding approaches and/or other alignment mechanisms. The absorption at the lower frequency part of the electromagnetic bands (below  1 - 2  GHz) can be improved via the addition of parallel lossy sheets into the low-density scaffolding.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Non-ProvisionalApplication Ser. No. 13/539,066, filed on Jun. 29, 2012, which claims abenefit, under 35 U.S.C. §119(e), of U.S. Provisional Application Ser.No. 61/502,842, dated 29 Jun. 2011, the entire contents and substance ofboth applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Anechoic chambers consist of shielded rooms with their interior surfaceslined with pyramidal or wedge shaped absorber. The absorber is designedto minimize the reflectivity of the wall over a wide range of radiofrequencies (RF). Traditional anechoic chamber absorber consists ofpolymeric foam loaded with conductive carbon, which provides goodnear-normal incidence absorption of specular energy. The length of theabsorber is related to the frequency over which it provides goodabsorption, and the thicker the absorber, the lower in frequency it canabsorb. For example absorber that is 6 feet (1.8 m) tall can providesignificant absorption down to approximately 200 MHz, while 18 inch (46cm) tall absorber works only down to 1 GHz.

This conventional material is fundamentally limited because of diffusescatter (tip diffraction) that increases the overall noise level in achamber. Diffuse scatter, unlike specular reflection, occurs in manydifferent directions for a given incidence of the RF energy. Tipdiffraction is directly related to the periodicity of the pyramidalabsorber, and occurs because the inherent inhomogeneity of a patternedstructure with a periodicity that is large relative to the wavelength ofthe incident RF energy. For example, in typical 18 inch tall absorber,the pyramids are 6 inch square (i.e. each 2-foot square piece ofabsorber has a 4 by 4 array of pyramids) and diffraction effects canoccur at frequencies above 1 GHz. Diffraction modes are also calledgrating modes or Floquet modes, and their angular position can bepredicted with a diffraction equation. For a simple 1-dimensionalgrating the diffraction equation is, d(sin θ_(m)+sin θ_(i))=mλ, where λis the wavelength, θ_(i) is the incident angle, θ_(m) is the diffractedangle, d is the periodicity.

Because the possible diffraction modes are only dependent on periodicity(i.e. d), no amount of reshaping of the pyramids will move these modes.The only way to reduce the number of diffractive modes is to reduce theperiodicity. This is why some anechoic chamber use wedge shapedabsorber, rather than pyramids to minimize diffraction. The disadvantageof wedge absorber is that the 1-dimensional geometrical taper is not aseffective as the 2-dimensional taper of the pyramids, so it is lesseffective in reducing specular reflectivity. Design of absorber in ananechoic chamber then becomes a compromise between specular performanceand diffraction effects.

SUMMARY OF THE INVENTION

The present invention is directed to an anechoic chamber absorber andmethod for constructing the anechoic chamber absorber that has improvedperformance over conventional pyramidal absorbers, while simultaneouslyminimizing or eliminating diffuse scatter due to diffraction effects. Toaddress this diffraction effect without reducing the specularperformance, the present invention relies on a periodicity that is muchsmaller than the wavelength so that the onset of diffraction occurs wellabove the frequency band of interest. When the periodicity is smallerthan half a wavelength, then it is referred to as electrically small,whereas a periodicity that is larger than a half wavelength isconsidered to be electrically large. Like pyramidal absorber, geometrictapering is used to obtain maximum specular absorption performance, butunlike pyramidal absorber, the geometrical or material tapering isapplied within this electrically small periodicity. The relatively poorstrength of pyramidal foam requires that pyramidal absorber is limitedto geometrical shapes that can be self-supporting, which are necessarilyelectrically large. However, in the present invention, a different setof constitutive materials is used to enable the necessary tapers to beapplied within an electrically small periodicity.

The anechoic chamber absorber suitable for absorbing electromagneticradiation is made using resistive sheets that have been rolled or foldedinto elongated tubes. Resistive sheets may be made from a thinconductive layer supported on a dielectric substrate layer. Any suitablerestive material may be used for the resistive layer, including thinsputtered or evaporated metal alloys, carbon loaded inks, or carbon ormetal embedded in a polymer matrix. The specific electromagneticproperties of these thin sheets are usually specified in terms of sheetimpedance, which is in units of ohms per square. Typical values ofresistive sheets for this application may range from a few ohms persquare to thousands of ohms per square depending on the performancerequirements. One low cost supply for resistive sheets is commerciallyavailable window tint; while the material is typically described interms of amount of light transmission, the material is oftenelectromagnetically lossy and can be used to form these elongated tubes.

Each of the individual elongated tubes has a front side and a back side(See FIG. 6), where the back side (i.e. metal backed 10) is placedagainst the walls of the anechoic chamber and the front side faces (i.e.physically tapered 3) towards the inside of the chamber. The diameter orwidth of the tube should be no more than a half of a wavelength in sizefor the frequency band of interest. The front side of each resistivetube can be tapered either geometrically, or with varying resistivity toprovide a gradual change in resistivity from the front of the tube tothe middle or back of the tube. Wth a geometric taper, the tube issimply cut with a slant cut so that it forms a point at the front side.Performance can be further improved with more elaborate physical shapingof the end.

The anechoic absorber is constructed by assembling the tapered tubesinto a periodic array with a periodicity or spacing that is preferablyless than a half-wavelength in the frequency band of interest. Thiselectrically small spacing has the advantage of preventing diffusescatter modes from occurring, providing an absorber that is inherentlyquieter (i.e. less diffuse and specular scattering) than traditionalpyramidal absorber. Another advantage of this invention is the hollowconstruction of the tubes. This hollow construction provides a verylight-weight construction for easier mounting of the absorber onto wallsor ceilings. Additionally, it provides a mechanism for convectivecooling in high-power applications, where the absorber can be activelycooled.

Other features and advantages of the invention will appear from thefollowing description in which the preferred embodiments have been setforth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a block of the present invention showinga multiplicity of periodic lossy elements with physically tapered ends.

FIG. 2 is a perspective view of a single unit cell of a lossy elementswith a metal base that is used to insure uniformity of spacing.

FIG. 3 is a perspective view of a single unit cell where the lossyelement is formed by rolling a resistive sheet with a metal base that isused to insure uniformity of spacing.

FIG. 4 is a perspective view of a single unit cell where the lossyelement is formed by folding a resistive sheet with a metal base that isused to insure uniformity of spacing.

FIG. 5 is a perspective view of a single unit cell where the lossyelement is formed by cutting and placing a lossy sheet on a low densitycarrier with a metal base that is used to insure uniformity of spacing.

FIG. 6 is a perspective view of a block of the present invention showinga multiplicity of periodic lossy element with a low density supportscaffold used at the base to maintain the lossy elements in a fixedarray.

FIG. 7 illustrates a plot of reflectivity versus frequency for an 18inch thick absorber, including the calculated reflectivity according tothe present invention and measured reflectivity for traditional carbonloaded pyramidal foam.

FIG. 8 shows a comparison of the allowed scattering grating lobedirections between the traditional pyramidal absorber (left hand side)to the present invention (right hand side) at two frequency bands.

FIG. 9 illustrates a plot of reflectivity versus frequency for a 36 inchthick absorber according to the present invention compared to acomparable 18 inch design.

FIG. 10 is an illustration of square lattice and hexagonal latticeconfigurations for the present invention.

FIG. 11 is a perspective view of a block of the present inventioncombined with three parallel resistive sheets tuned to extend the lowfrequency performance.

FIG. 12 illustrates a plot of reflectivity versus frequency for an 18inch thick absorber with just restive tubes compared to a similarabsorber with both resistive tubes and parallel resistive sheets in thelow density structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows one of the simplest preferred embodiments. Lossy elements 1are placed periodically at the sites of a lattice formed from twonon-coincident lattice directions 2, denoted S and P. FIG. 1 shows thecase where S and P are orthogonal forming a rectangular lattice. Thelossy elements 1 have physically tapered 2 ends to better absorb theincident electromagnetic energy.

In the preferred embodiments the physically tapered 2 ends are a simplelinear bevel as shown in FIG. 1. In alternate embodiments the curvatureof the physical shaping would be optimized using computationalelectromagnetic tools.

FIG. 2 shows an embodiment for forming the lossy elements 1 with metalbases 4 where the size of the metal base 4 matches the periodic lattice2 in shape and size. These metal bases 4 of the lossy elements 1 whenplaced in the periodic arrangement would form a continuous metalbackplane. Also, these metal bases 4 would provide a means for insuringthe proper periodic spacing and alignment. In some embodiments the lossyelements 1 can be formed from classical absorber materials such asvolumetrically lossy materials like carbon loaded foam as long as theperiodic elements are disconnected from each other.

Notice, that one unique feature of this new absorber is the disconnectednature of the periodic elements. Classical absorbers such as lossypyramidal foam and carbon loaded honeycomb structures are connectedlossy structures. Disconnecting the lossy elements fundamentally changesthe frequency dependent nature of the loss mechanism which is fullymodeled in our design approach and leads to scalable manufacturing.

FIG. 3 further refines the embodiment by showing that the lossy elements1 are formed by rolled resistive sheets 5. FIG. 4 shows an alternateembodiment where the lossy elements 1 are formed by a folded theresistive sheets 6. Alternate embodiments include the use of magramsheets. The lossy elements 1 would be formed from rolled or foldedmagram sheets in these alternate embodiments. Even combinations ofresistive and magram sheets are envisioned where one is rolled insidethe other. Each material has advantages and disadvantages. The resistivesheets are the lightest weight choice and are commercially available asresistive cards (Rcards). The magnetic (magram) materials are heavierbut also are commercially available in sheet form. Magram isparticularly attractive at absorbing lower frequency electromagneticenergy (below 1-2 GHz) and thus depending on the end-user requirements,the lossy elements 1 may be best made by trading between resistiveand/or magram sheets.

FIG. 5 shows one more embodiment for physically realizing lossy elements1. In this embodiment, a low density carrier 7 is used to carry cut andplaced lossy sheets 8. The cut and placed sheets 8 can be placed on oneor more sides of the low density carrier 7. Depending on the materialproperties of the materials designs may meet the absorption targetsusing less all the surfaces. However, to absorb both polarizations ofelectromagnetic energy at least two non-parallel sides should be used.Best performance may come from using all four sides at the addition ofmore complexity. Notice the both the low density carrier 7 and cut andpasted lossy sheets 8 have physically tapered 3 ends. The need tophysically taper 3 ends is required on both lossy and non-lossymaterials in order to meet the greatest levels of absorption that arebecoming commercially required.

FIG. 6 shows an alternate embodiment where a low density structure 9 isused to support the lossy elements 1 on the periodic lattice 2. The lowdensity structure 9 can be formed from low density materials such asfoam and honeycomb. For solid materials like foam, periodic holes 12would be formed using water-jet cutting or similar technology. Fornon-solid materials like honeycomb, the preferred embodiment would havethe honeycomb lattice match the desired lattice of the absorber butalternatively the lattice could be smaller and holes could be cut at therequired sites. Depending on end-user usage, the low density structure 9may be optionally metal-backed 10 to insure consistent performance andprovide further shielding. This metal backing can be formed easily fromvarious metal foils to keep the mass low or from thicker metal honeycombto add structural properties.

FIG. 7 shows a performance prediction for an 18″ thick design where thelossy elements 1 are formed into 1.4 cm diameter elongated tubes fromfolded resistive sheets 6. The lossy elements are arranged on arectangular lattice of 2 cm spacing. A series of electromagneticsimulations are performed using state-of-the-art computationalelectromagnetic codes to optimize the properties of the folded resistivesheets 6 and the length of the physically tapered 2 ends. The solid linein FIG. 6 shows the absorption of a sample design compared withconventional 18-inch thick pyramidal absorber (dashed line) showing thesuperior performance possible from this new approach to absorber.

FIG. 8 shows the improvement in diffuse scatter for this new approach toabsorber. The plots on the right hand side show the allowed scatteringdirections for the new, low diffuse scatter absorber that is the subjectof this utility patent compared to the conventional pyramidal absorbershown on the left hand side. The top figures show the modes for 8-12 GHzand the bottom figures show 12-18 GHz. Each black dot is are thelocation of a potential scattering grating lobe. The size of thetraditional pyramidal absorber has a tremendous number of allowedscattering grating lobe directions as evidenced by the near solid blackcoverage of the left hand side plots. The new, low diffuse scatterabsorber has a periodicity of 2 cm and thus there are no allowed gratinglobe scattering directions until above 12 GHz as evidence by the singlespecular scatter black dot in the top right plot and only a smallernumber of possible grating scatter directions from 12-18 GHz in thebottom plot. Clearly the ability to form absorber using disconnectedtubes allows us to choose the tube diameter and lattice spacing toeliminate or greatly reduce the number grating lobes up to desiredfrequencies.

FIG. 9 shows how the performance of the folded resistive sheet 6embodiment can be further improved by lengthening the tubes. The dashedline is an optimized design for 36-inch length compared to a comparable18-inch design (solid line). The 36-inch design is clearly superior.Hence, with this new approach to absorber, various length lossy elements1 can be fashioned to meet the levels of desired performance of eachcustomer.

Alternate embodiments can utilize other periodic arrangements of lossyelements 1. FIG. 10 shows two preferred arrangements. The drawing on theleft shows the square lattice arrangement where the lattice directions 2are perpendicular and the lattice spacings are equal. The drawing on theright shows the hexagonal lattice arrangement where the latticedirections 2 are separated by 60 degrees and the lattice spacings areequal. Other embodiments are envisioned using other lattice spacings butthe choice of lattice would be largely chosen based on the topology andsize of the area to be covered with absorber and the spacing would bechosen to meet the electromagnetic diffuse scatter requirements.

FIG. 11 shows an alternate embodiment with improved performance. Theperformance improvement comes from the addition of one or more lossysheets 9 into the low density structure 9. The material properties andplacement of these lossy sheets 11 are optimized using a series ofelectromagnetic simulations are performed using state-of-the-artcomputational electromagnetic codes. FIG. 12 shows how the addition ofthese lossy sheets 11 allows improved low frequency (below 1-2 GHz)absorption.

This embodiment shows how we can tailor the loss to best meet end-userrequirements. For example, typically the high frequency (above 1-2 GHz)absorption is typically good enough for a typical thickness of 18-36″but often the low frequency absorption is not enough. By including anumber of parallel lossy sheets into the low density support, the lowfrequency performance can be improved somewhat independently of the highfrequencies. Of course, for final design would include all the detailsin the computational model to ensure absorption across the fullfrequency band.

Therefore, at least the following is claimed:
 1. A method of making ananechoic chamber absorber comprising placing a periodic arrangement ofdisconnected, electromagnetically lossy elements embedded into anelectromagnetically lossy structure on a lattice described by twonon-coincident directions, the electromagnetically lossy elements arefree-standing tubular structures having a first section embedded intothe electromagnetically lossy structure at a proximal end and extendingoutward to a distal end and a second section extending from the distalend of the first section to a distal end of that electromagneticallylossy element, where a cross-sectional shape of the tubular structure isconstant along an axial length of the first section and is tapered alongan axial length of the second section, where the second section istapered along a plane that extends from one side of the tubularstructure at the distal end of the first section to an opposite side ofthe tubular structure at the distal end of that electromagneticallylossy element, where no structures are disposed between adjacent firstsections extending outward from the electromagnetically lossy structureand between adjacent second sections of adjacent electromagneticallylossy elements in the periodic arrangement.
 2. The method of claim 1where the electromagnetically lossy elements are embedded perpendicularto an outer surface of the electromagnetically lossy structure.
 3. Themethod of claim 1 where the electromagnetically lossy structure isformed from one or more parallel resistive sheets separated by a lowdensity material.
 4. The method of claim 1 where the electromagneticallylossy structure is formed from one or more parallel magram sheetsseparated by a low density material.
 5. The method of claim 1 where theelectromagnetically lossy structure is formed from a combination ofparallel resistive and magram sheets separated by a low densitymaterial.
 6. The method of claim 1 where a periodic lattice of holes isformed in the electromagnetically lossy structure to allow placement ofthe electromagnetically lossy elements into the electromagneticallylossy structure.
 7. The method of claim 6 where the two non-coincidentdirections of the periodic lattice of holes are orthogonal forming arectangular array of holes.
 8. The method of claim 6 where the twonon-coincident directions of the periodic lattice of holes are separatedby 60 degrees forming a hexagonal arrangement of holes.
 9. The method ofclaim 1 where the electromagnetically lossy elements are formed byrolling resistively coated sheets into tubes.
 10. The method of claim 1where the electromagnetically lossy elements are formed by rollingmagnetic ram sheets into tubes.
 11. The method of claim 1 where theelectromagnetically lossy elements are formed by foldingelectromagnetically lossy sheets into tubes.
 12. The method of claim 1where the electromagnetically lossy elements are formed by cutting andplacing shaped pieces of electromagnetically lossy materials onto lowdensity carriers.
 13. The method of claim 1 where theelectromagnetically lossy elements are formed from window tintmaterials.
 14. The method of claim 1 where the electromagnetically lossyelements are physically tapered by shaping the ends.
 15. The method ofclaim 1 where the electromagnetically lossy elements are formed fromcoincident tubes made from resistively coated sheets and magram sheets.16. The method of claim 1 where the anechoic chamber absorber isconfigured to absorb electromagnetic waves in a frequency range havingan upper frequency with a wavelength, wherein the periodic arrangementhas a lattice spacing of the disconnected, electromagnetically lossyelements that is equal to or less than one-half of the wavelength of theupper frequency.
 17. An anechoic chamber absorber comprising a periodicarrangement of disconnected, electromagnetically lossy elements placedon a lattice described by two non-coincident directions, where theelectromagnetically lossy elements are free-standing tubular structureshaving a first section extending from the lattice at a proximal end anda second section extending from a distal end of the first section to adistal end of that electromagnetically lossy element, where across-sectional shape of the tubular structure is constant along anaxial length of the first section and is tapered along an axial lengthof the second section, where the second section is tapered along a planethat extends from one side of the tubular structure at the distal end ofthe first section to an opposite side of the tubular structure at thedistal end of that electromagnetically lossy element, where nostructures are disposed between adjacent first sections and betweenadjacent second sections of adjacent electromagnetically lossy elementsextending from the lattice in the periodic arrangement.
 18. The anechoicchamber absorber of claim 17 where the proximal end of the first sectionof the electromagnetically lossy elements extend through anelectromagnetically lossy structure on the lattice.
 19. The anechoicchamber absorber of claim 17 where the two non-coincident directions areseparated by 60 degrees forming a hexagonal arrangement, whereinelectromagnetically lossy elements immediately adjacent to one of theelectromagnetically lossy elements are spaced at 60 degree intervalsthereby forming the hexagonal arrangement.
 20. The anechoic chamberabsorber of claim 17 configured to absorb electromagnetic waves in afrequency range having an upper frequency with a wavelength, wherein theperiodic arrangement has a lattice spacing of the disconnected,electromagnetically lossy elements that is equal to or less thanone-half of the wavelength of the upper frequency.